According to Swiss newspaper Tages Anzeiger, two independent sources have confirmed that ultrasonic testing has shown there are around 1,000 holes, or cavities, of half a centimetre in diameter within the steel walls of the reactor's pressure vessel.
These vessels contain nuclear fuel and are made of thick steel plates welded together. But neutrons from the fuel irradiate the vessel during operations, weakening the steel over time – a process known as embrittlement.
This is particularly true of pressurized water reactors like Switzerland's Beznau I, and is more pronounced in situations where poorer quality steels – those with a higher proportion of copper and nickel for example – have been used.
Picking up the story, media outlets including Swiss-French daily Tribune de Genève dubbed the reactor “Emmental, or nearly”, after the famously holey cheese.
The new information means the situation at Beznau I could be far worse than the plant's operator, energy utility Axpo, declared back in July when it first detected signs of "minimal irregularities in the production process" of the reactor's pressure vessel during its annual resting period.
“They aren't cracks, but possibly areas of dirt or variations in the thickness of the material in certain places,” the energy company is reported as saying at the time.
That assessment came after the Western European Nuclear Regulators' Association, Wenra, recommended all forged reactor pressure vessels in Europe be inspected after problems were found with two Belgian reactors.
The Swiss Federal Nuclear Safety Inspectorate responded by calling for more detailed assessment, and said: "The operation of Beznau 1 may only be started again if the safety of the reactor pressure vessel is confirmed."
The affected Beznau pressure vessel is where the nuclear process takes place, said Tages Anzeiger, meaning there is a risk that highly radioactive material could escape.
Beznau I has remained offline since July but Axpo currently plans to restart it next February.
However an insider told the Tages Anzeiger that the scale of the problem is so serious that the reactor may never resume service.
Built in 1969, Beznau I is the oldest working nuclear reactor in the world.
Located on an artificial island on the Aare river, Beznau is just 37 kilometres from central Zurich and 10 kilometres from the German border.
The second reactor on the complex was built in 1971 and is also currently in a four-month resting period.
On October 4th a study commissioned by a Swiss anti-nuclear body classed Beznau as the fourth most dangerous nuclear reactor in the world out of 194 studied.
In August environmental campaign group Greenpeace launched legal action to demand that Beznau I be shut down, insisting it would not resist a powerful earthquake.
In 2012 a government-commissioned study found that all four of Switzerland's nuclear reactors were fit to withstand an earthquake.
But Greenpeace said the findings were based on “erroneous technical norms.”